1. Field of the Invention
The present invention relates to an apparatus for controlling excitation current to the field winding of a generator or motor, and more particularly to a circuit which uses an uncontrolled current source and a controlled current source to trim excitation current to a field winding.
2. Background of the Invention
In the drawings referenced herein, like numerals indicate like features.
Motors and generators can be classified as brush-type or brushless. In a brush-type machine, electrically conductive brushes connected to slip rings provide excitation current to a rotating field winding.. In brushless machines, excitation current is provided to an excitation field winding. Rotating inductors convert the magnetic flux created by the excitation field winding into current supplied to the rotating field winding, The present invention will be described in terms of a brush-type system. It will be obvious to one skilled in the art to apply the teachings of the present invention to a brushless motor or generator.
A typical brush-type system 10 is shown in FIG. 1. System 10 comprises a rotating field winding 12 comprising winding ends 24 and 26, a stationary (main) winding 14 and an automatic voltage regulator (AVR) 18. Magnetic flux created by rotating field winding 12 is converted into an AC voltage supplied to a load 16 by main winding 14. AVR 18 controls the voltage supplied to load 16 by increasing or decreasing the magnetic flux generated by winding 12 as a function of the voltage sensed across winding 14. In a typical system, the voltage supplied to load 16 is controlled by sensing the voltage across winding 14 and supplying an excitation current as a function of the voltage sensed to winding 12 through the slip rings (not shown).
A typical AVR circuit is shown generally in FIG. 2. In FIG. 2, two silicon-controlled rectifiers (SCR) 32 and 34 and two diodes 36 and 38 form a two-pulse half-controlled bridge converter capable of converting an AC voltage into a DC voltage used to control excitation current to rotating field winding 12. SCRs 32 and 34 are controlled by excitation current control 30.
SCR's are a well known and used method of controlling field current for a voltage regulator in an AC generator or a DC generator. Excitation current control 30 controls the excitation current provided to winding 12 by increasing or decreasing the turn-on time of SCRs 32 and 34. This increases or decreases the average DC current provided to winding 12 which, in turn, increases or decreases the magnetic flux generated by winding 12. An AVR constructed as in FIG. 2 is useful in the control of AC generators in widely varying conditions and under widely varying loads.
A second type of voltage regulation is shown as system 40 in FIG. 3. In FIG. 3, a center tapped winding 42 replaces winding 14 Of FIG. 1. System 40 comprises a rotating field winding 12, a center-tapped stationary (main) winding 42 and an automatic voltage regulator (AVR) 44. Magnetic flux created by rotating field winding 12 is converted into an AC voltage supplied to a load 16 by main winding 42.
AVR 44 controls the voltage supplied to load 16 by increasing or decreasing the magnetic flux generated by winding 12 as a function of the voltage sensed across winding 42. In atypical system, the voltage supplied to load 16 is controlled by sensing the voltages between winding ends 20 and 22 and center tap 46. An excitation current is provided to winding 12 as a function of the voltages sensed.
A typical AVR circuit for system 40 is shown generally in FIG. 4. In FIG. 4, two silicon-controlled rectifiers (SCR) 50 and 52 form a two-pulse midpoint converter capable of converting an AC voltage into a DC voltage used to control excitation current to rotating field winding 12. SCRs 50 and 52 are controlled by excitation current control 54. Excitation current control 54 controls the excitation current provided to winding 12 by increasing or decreasing the turn-on time of SCRs 50 and 52. This increases or decreases the average DC current provided to winding-12 which, in turn, increases or decreases the magnetic flux generated by winding 12. Like the AVR shown in FIG. 2, an AVR constructed as in FIG. 4 is useful in the control of AC generators in widely varying conditions and under widely varying loads.
AVRs 18 and 44 provide feedback control over the magnetic flux generated by winding 12, in a generator or motor, but at a cost. An AVR design based on active components such as SCRs is generally more costly than a purely passive design. For one thing, an SCR is more expensive than a diode. In addition, each SCR requires the addition of the support circuitry needed to turn the SCR on and off as a function of the excitation current required. Passive diodes cannot, however, be used in the place of the SCRs shown in FIGS. 2 and 4 without relinquishing control over the excitation current.
In the case of stand-alone generators, AVRs based on active components such as SCRs face an additional problem. Since the AVR needs power to turn on its SCRs, no excitation current is provided until the voltage generated is sufficient to power the SCRs. To Counter this, stand-alone generators must either be provided with an independent energy source to power the active components when the generator is first turned on or the AVR must be designed to remain quiescent for the time necessary for the residual voltage to reach the level needed to power the active components. In the latter case, the generator operates without control while the generated voltage ramps up. This may require additional circuitry for field flashing.
It is desirable to minimize the number of active components in an AVR both to minimize cost and design complexity. What is needed is a method of using passive diodes to replace one or more of the SCRs in a automatic voltage regulator while maintaining the control necessary to trim the voltage supplied to a widely varying load. The present invention meets that need.